Films having tailorable surface roughness and/or a low coefficient of friction have been long sought. Such films are useful in a wide variety of applications including packaging, printing, electrical insulation, capacitor manufacture, backings for adhesive tapes, magnetic recording tapes and discs, computer tapes, writeable surfaces, and the like.
The low coefficient of friction is desirable so as to improve handling properties of the film during manufacture and use and to prevent blocking during storage. The tailorable surface roughness is desirable so as to provide appropriate surface structure to the film. For instance, substrates employed in magnetic recording media must be relatively smooth on the surface to which the magnetic coating is applied. On the other hand, the running or opposite side of the magnetic substrate, must have a characteristic that resists abrasion from contacting surfaces such as tape drive mechanisms. Capacitor films and writeable films also must have a high surface roughness to respectively allow oil impregnation and to accept ink or pencil, etc.
Several approaches have been employed in order to provide a film with surface roughness and a low coefficient of friction. Generally they involved the use of the so called slip agents (e.g., inorganic particulates and organic materials which do not dissolve in the host polymer), low coefficient of friction coatings, surface texturing and the like.
Each of these approaches suffers from one or more disadvantages. For example, the inorganic particulate slip agents (e.g., silica, talc, mica, glass, calcium carbonate, titanium dioxide, etc.) and the previously used non-soluble organic material slip agents (e.g., crosslinked polymers, combinations of fluorocarbon polymers with fatty acid amides, polycarbonates, polyolefins, etc.) may clog the filtration devices used in the manufacture of the film. Additionally, such slip agents may be present as undesirably large agglomerates in the film which negatively effect certain applications such as magnetic recording media.
The use of these slip agents suffers from other disadvantages. The incorporation of inorganic particulates usually requires that they be milled to the appropriate size. This is an added step that is difficult to control, provides particles of widely varying size, (resulting in unpredictable surface roughness in the film), and adds cost. The use of the non-soluble organic materials usually requires a large loading and makes recycling the film difficult.
The application of low coefficient of friction coatings (e.g., silicone coatings) to the surface of the film also suffers from various disadvantages. For example, such coatings generally are not durable, the application of such coatings complicates the manufacturing process and adds cost, and many of such coatings detrimentally affect the adhesion of the film to subsequently applied materials.
Surface texturing of the film is usually achieved by external means such as treatment of the surface with sputter etching, and the like. Such techniques also complicate the manufacturing process and add cost.
A film has now been discovered which overcomes these disadvantages of the prior art. The film not only possesses the unique characteristic that its surface roughness can be tailored to suit the users needs, it also preferably has a low coefficient of friction. Furthermore, manufacture of the film does not quickly clog the filtration devices used during such manufacture thus extending the useful life. These results are achieved by incorporating a thermotropic liquid crystalline polymer (sometimes referred to hereinafter as LCP) with a host polymer.
The use of LCP's with other polymeric materials has been previously suggested. See, for example, U.S. Pat. No. 4,442,057 in which a combination of a fiber-forming polymer and a small amount of a polymer capable of forming an anisotropic melt (an LCP) is melt spun at a minimum windup speed of 1,000 meters per minute. This patent is directed solely to the proposition that the LCP provides windup speed suppression (i.e., the properties of the spun fiber are those that would be obtained from a fiber spun at a lower windup speed).
Other patents which disclose the use of LCP's with other polymeric materials include United Kingdom Patent No. 2,078,240 which discloses the use of from 25 to 95 percent by weight LCP with the other polymer; U.S. Pat. No. 4,408,022 which discloses the use of from 25 to 50 percent by weight LCP with one or more additional polymers; U.S. Pat. No. 4,451,611 which discloses the use of from 85 to 95 percent by weight LCP; and EPO Patent No. 169,947 which discloses the use of from 20 to 80 percent by weight LCP in combination with another polymer. These patents are each directed to blends of polymers useful as injection molding resins. Although some also state that the blends can be used in the formation of fibers and films, nothing is stated with respect to the formation of low coefficient of friction films having a tailorable surface roughness, oriented films of this type, or the formation of discrete regions of the LCP in a matrix of the host polymer.
Still other publications which disclose the use of LCP with another polymer are EPO Patent Application 0 071 968 which discloses a thermoplastic composition containing an isotropic thermoplastic material and an oligomeric thermotropic (liquid crystalline in the melt additive); and Japanese Kokai JP 61-78862 and JP 61-78863 which disclose a biaxially oriented film respectively containing 1-60 weight percent and 1-15 weight percent liquid crystalline material in a matrix polymer. The EPO publication is silent with respect to the formation of discrete regions of the LCP and the host polymer. Additionally, none of these publications disclose low coefficient of friction properties or a film having a tailored surface roughness.
Moreover, films disclosed in the two Japanese publications are said to possess improved bulk properties, such as improved elastic modulus, impact resistance, and dimensional stability, due to the formation of acicular (i.e., needle-like or rod-like regions) of the LCP in polyester. The LCP regions have a high aspect ratio which results from employing a draft ratio (i.e., degree of melt stretching) of 3-30 times in the film manufacture.